Apparatus is known to the prior art for measuring the electrical impedance of particulate materials, such as wood chips, grain, hogged fuel, sawdust, coal, and various ore concentrates. In the particular case of wood chips, such apparatus may include a sample box for receiving and retaining a sample of wood chips during measurement, the sample box defining a pair of electrodes which form part of an electrical bridge circuit which is energized by a high frequency, alternating current signal. The bridge circuit provides an output signal which is related to the electrical impedance of the wood chips contained within the sample box. After compensation for temperature and bulk density variations of the wood chips, the bridge output signal is then rectified in a controllable manner to provide a DC output signal which is related to a selected component, either resistive or reactive, of the measured electrical impedance.
Illustrative examples of such impedance measuring apparatus can be found in U.S. Pat. No. 3,781,671, issued Dec. 25, 1973 to Fritz K. Preikschat, and entitled IMPEDANCE MEASURING BRIDGE CIRCUIT; U.S. Pat. No. 3,824,461, issued July 16, 1974 to Fritz K. Preikschat, and entitled ELECTRICAL IMPEDANCE MEASURING APPARATUS; and U.S. Pat. No. 3,992,665, issued Nov. 16, 1976 to Fritz K. Preikschat and entitled ELECTRICAL IMPEDANCE MEASURING APPARATUS. Each of the foregoing patents is expressly incorporated by reference herein.
It is particularly desirable to accurately measure the moisture content of wood chips, since that moisture content is an important factor in determining the liquor-wood ratio used in processes for making pulp from wood chips. Heretofore, it has been recognized that the dielectric coefficient of wood chips is directly related to moisture content. Therefore, the electrical impedance measuring apparatus of the prior art has been operative to measure the reactive component of the measured electrical impedance, and to thereby provide an output signal which is related to the measured dielectric coefficient of the wood chips.
It has become increasingly evident, however, that the measured dielectric coefficient is not in all cases a true measure of the moisture content of wood chips. The reasons for this lack of correlation between measured dielectric coefficient and moisture content can usually be traced to variations in the physical characteristics of the wood chips. The prior art, as typified by the aforementioned patents, recognizes that both temperature and bulk density variations in the wood chips can affect the dielectric coefficient measurement, and therefore provide means to compensate for such variations. The present invention is based on the recognition that there are certain other factors that also affect the dielectric coefficient measurement and which must be taken into account in order to obtain an accurate measurement of moisture content.
For example, a sample of wood chips typically includes a varying percentage, by weight, of what are termed "fines," or, wood particles such as sawdust which have an average size less than 1/4 inch screen mesh. FIG. 1 represents a typical plot of the variation in measured dielectric coefficient, which in turn is related to the reactive component of the measured electrical impedance, and the variation in measured conductivity, which in turn is related to the resistive component of the measured electrical impedance, with variation in the percentage of fines by total weight in a sample of chips. It will be seen that both the measured dielectric coefficient and the measured conductivity increase with an increase in the percentage of fines, and that an increase in the percentage of fines of approximately 10% results in an increase in measured conductivity of approximately 50% and an increase in measured dielectric coefficient of approximately 5%. It has been postulated that the increase in measured conductivity is due to a more effective surface-to-surface contact between the individual chips provided by the increased amount of fines, and that the increase in measured dielectric coefficient is related to the increase in conductivity, inasmuch as the measured dielectric coefficient is that of an equivalent circuit including the actual dielectric coefficient of the wood chips in parallel with the actual conductivity between wood chips.
As yet another example, the logs used to produce wood chips are in many cases stored in salt water whose ionic conductivity may vary. As a result, the conductivity of a sample of wood chips may also vary so that the measured dielectric coefficient is not truly representative of the moisture content of the wood chips.
As yet another example, when wood chips are held in storage, they are subject to decomposition largely caused by fungi attack, which through the action of enzymes, causes a breakdown in the cellulosic components of the chips. If the temperature of the chips exceeds 70-75.degree. C., an additional deterioration occurs through oxidation and respiration effects. As a result, acetic acid is formed which increases the conductivity of the chips to similarly increase the measured dielectric coefficient.
It is therefore an object of this invention to provide an electrical impedance measuring apparatus which is capable of more accurately determining the moisture content of particulate materials than the electrical impedance measuring apparatus of the prior art.
In pulping processes, it is often important to know when wood chips being processed have an unusually high percentage of fines, or contain an unusually high percentage of salt water, or are significantly degraded. For example, the fines in wood chips will absorb the active alkali in a cooking liquor typically used in a pulping process more quickly than the large wood chips, thereby reducing the effective alkali concentration earlier in the process, with the result that increased amounts of alkali must be added to the liquor to obtain a uniform pulp. As another example, degradation of wood chips results in a higher than normal proportion of chemical components that are active in the alkali which causes an excessive liquor consumption and reduction in yield. Also, the acetic acid produced as a result of chip decomposition requires increased amounts of alkali in the liquor. Since conductivity is a measure of the fines content and the amount of degradation of a given sample of wood chips, it is desirable to measure conductivity directly to provide information that can be used in the control of a pulping process in order to obtain a more uniform pulp product.
It is therefore a further object of this invention to provide an electrical impedance measuring apparatus which is capable of providing separate measurements of both the conductivity and dielectric coefficient of particulate material.